Grouting is the placement of a suitable, low hydraulic conductivity material into the annular space between the well casing and borehole, or between the existing casing and liner pipe. The main purposes of grouting and sealing a well are to (1) restore the earth formation outside the casing to its original condition, (2) prevent seepage of polluted surface water downwardly along the exterior of the casing into the well, (3) stabilize and secure the well casing, and (4) preserve the hydraulic characteristics of artesian formations and prevent leakage upwardly along the exterior of the casing. Basically there are three types of grouting material which can be used to serve these purposes. They are (1) cement-based, grouting materials, (2) bentonite-based grouting compositions, and (3) geological materials such as glacial till and sandstone. A suitable grouting material should be able to provide a good seal having both low hydraulic conductivity and high structural stability under most geological conditions. Bentonite-based grouting compositions are known to exhibit low permeability and high swelling capability as compared to cement-based grouting materials. In bentonite-based compositions, the degree of impermeability to aqueous solutions with low salinity is directly related to not only the amount of bentonitic clay, but also to the quality of the clay. For example, sodium-based bentonite or sodium montmorillonite has a much higher swelling pressure than calcium-based bentonite. Because of its high swelling capability, sodium montmorillonite clay particles, when in contact with water, swell immediately and expand the clay lattice. Therefore, the grouting compositions which contain high sodium-based bentonite without addition of a clay thinner or dispersant in either direct or indirect manner tend to stick together and become lumpy when the composition is added to fresh water. The resultant slurry becomes excessively viscous and too thick to pump. This is particularly the problem for those finely divided sodium montmorillonite particles with particle size distribution below 75 micrometers (200 mesh sieve). Such a thick slurry having a very short working time or set time will not only clog pumping equipment, but will also impede the well grouting operation.
U.S. Pat. No. 4,964,918, to Richard K. Brown et al. describes a grouting composition containing 98-99.9% of finely divided, water swellable, colloidal clay such as bentonite, and 0. 1-2% of a synthetic organic polymer, polyacrylate, used as a clay thinner. The grouting composition can be added directly into freshwater to provide a grout admixture having 30-35% reswellable solids. A clay thinner can be simply a deflocculant which dissociates clay aggregates physically to reduce slurry viscosity and increase fluidity. The particular thinner disclosed in the '918 patent is an anionic organic polymeric compound containing highly negatived charged sites which prevent clay particles having a net negative charge from associating with each other, thus making the grouting slurry thinner.
U.S. Pat. No. 4,964,918 to Richard K. Brown et al. further teaches that the water swellable clay should be finely ground, wherein at least 80% passes a 200 mesh U.S. Standard sieve. Their grouting composition, made up with finely divided water swellable clay particles, is more likely to provide a thicker slurry when added to freshwater. The finer or smaller the clay particles are, the more surface area is accessible to water adsorption, and the thicker the resultant slurry becomes.
U.S. Pat. No. 4,948,428 to Andrew W. Liao describes a grouting composition comprised of a water swellable granular bentonitic clay and an aqueous admixture containing a hectoride, an alkaline metal tetraborate, an alkaline metal carbonate, and an alkaline metal chloride, wherein the inorganic mixture functions as a clay dispersant. This process requires two separate steps to prepare the grouting slurry from the disclosed grouting compositions. The inorganic mixture is first added to fresh water to prepare an aqueous clay dispersion solution; then granular sodium bentonite is mixed with the dispersion solution to provide a high strength grout slurry.
U.S. Pat. No. 4,696,698 to Todd D. Harriett describes grouting compositions containing up to 80% non-swelling particulate fillers such as fly ash, diatomaceous earth or calcium-based clays, as well as up to 35% gelling agents such as magnesium oxide or soda ash and up to 35 % multivalent cation source and as an option, uses sodium pyrophosphate as a clay dispersant.
U.S. Pat. No. 4,463,808 to Carl E. Mason et al. describes a grouting composition in which granular sodium-based bentonite is added to a premixed organic polymer solution containing water dispersable partially hydrolyzed polyacrylamide synthetic polymer known as PHPA. This polymer retards granular bentonite clay particles from swelling and allows the grouting slurry thus produced to be placed at the desired location in the well borehole. Because of growing environmental concerns, use of the grouting materials containing synthetic organic polymers and sulfonated polymers of some aromatic compounds have become less acceptable in many areas of the world.